1. Field of the Invention
The invention relates to a method and device for determining the position of immersed objects with respect to a ship which is towing them.
More particularly, the invention relates to a method and device for determining the position of elements of a seismic transmission and reception device towed immersed by a ship, with respect to several locations whose position with respect to a ship is determined.
2. Description of the Prior Art
An acoustic telemetric method is known for determining the position of an object or a vehicle towed while immersed by a ship which consists in immersing transponders at well defined locations in the scheduled zone of evolution and in measuring, by acoustic telemetering, the distances between the towing ship, the transponders and the vehicle. The position of the towed object or vehicle with respect to the ship is calculated by triangulation. Such a method is described in the U.S. Pat. No. 4,229,809.
The accuracy which can be reached is good because the spacing between the transponders is of the same order of size as the distances to be measured.
Its use is however difficult because the zone of evolution must be marked out with beacons prior to immersion of the transponders.
This is an appreciable disadvantage especially when the marking out must be done specially for a particular application and the zone to be marked out is very extensive, as is the case particularly in seismic sea prospection.
Another known method of determining the position in the water of a towed object, in this case that of different points of a seismic streamer with respect to the towing vehicle, is described in the U.S. Pat. No. 4,376,301.
In this method, the distance is determined by acoustic telemetering between two acoustic pulse sources fixed to the ship and whose positions are accurately known and each hydrophone of a plurality of hydrophones spaced apart along the streamer.
The French patent application No. 2,545,226 also describes a method for determining the position in the water of a seismic streamer towed while immersed, consisting essentially in measuring the distances to the ship of a plurality of points spaced apart along the streamer and the azimuth of these points, by means of a gyro compass and a very short based hydro-acoustic system.
For essentially practical reasons, the spacing between the reference points where the acoustic transducers of the acoustic telemetering system are fixed with respect to the ship, is often very small with respect to the length of the towed objects, whether it is a question of a multisource transmitting device or a seismic streamer.
The distance between the reference points is in the range between a few tens of centimeters and a few meters whereas the streamer sometimes extends over several kilometers. The triangle formed by the reference points of the base and each point of the seismic streamer is disproportional, which adversely affects the accuracy of the telemetric measurements or of angular measurements made from the ship.
From the French application No. 2,533,706, a method is known of localizing an objet towed while immersed with respect to a towing ship. Parallel thereto and set back therefrom moves an independent auxiliary ship. The position of the two ships with respect to each other is measured by a conventional radio-navigational system and by triangulation by means of immersed acoustic transponders.
The position of the towed object (in this case a seismic streamer) is determined by triangulation by means of an acoustic telemetering system including a transmitter-receiver under the main ship, an acoustic receiver under the auxiliary ship and acoustic transponders spaced apart along the towed object.
The use of this method requires then the use of two independent ships each having a radio-navigational positioning system and a radio transmission assembly for transmitting to the main ship the signals indicative of the arrival times of the acoustic responses reaching the secondary ship. Furthermore, the acoustic transmitters and receivers associated with the ship are immersed at a relatively low depth and it can be seen that that may make the detection of acoustic signals difficult because of the disturbance of the water in its surface layers particularly in the wake.
From the patent application No. WO 84/03153, a location system is known for determining the position in the water of a seismic streamer with respect to a ship which is towing it. This system includes more particularly acoustic transponders fixed to towed paravanes which are offset laterally with respect to the counter of the ship. The position of the seismic streamer is in this case determined by acoustic telemetering with respect to these transponders.